This invention relates, in general, to semiconductor devices, and more particularly, to non-volatile memory devices.
Electrically erasable and programmable read-only memory (EEPROM) devices are one form of non-volatile memory devices which are erased and programmed using electrical signals. Within an EEPROM device are a plurality of memory cells, each of which may be individually programmed and erased. In general, an EEPROM cell includes a floating gate transistor and a select transistor. The select transistors in an EEPROM device are used to select individual EEPROM cells which are to be erased or programmed. The floating gate transistors in the device are those transistors which actually store the digital value of each particular memory cell.
To program and erase a cell, a phenomenon known as Fowler-Nordheim tunneling is commonly used to store either a positive or a negative charge on a floating gate electrode of the floating gate transistor. For example, programming is accomplished by applying a positive voltage to a drain and a gate of the select gate transistor while a control gate of the floating gate transistor is held at ground. As a result, electrons tunnel from the floating gate of the floating gate transistor through a tunnel dielectric to the drain, leaving the floating gate positively charged.
One limitation of conventional EEPROM devices is their inability to operate reliably under extreme temperature conditions. For example, in applications involving automotive or industrial conditions, EEPROM devices can be exposed to temperatures in excess of 135.degree. Celsius (C) or as low as -40.degree. C. Under these extreme temperatures conditions, both the data retention and the write/erase cycling (endurance) of the EEPROM device are severely degraded. This is due in part to the characteristics of the materials used to form the dielectric structures within the EEPROM device.
Accordingly, it would be advantageous to provide a non-volatile memory device that can operate under elevated temperatures and still maintain desirable endurance and data retention characteristics.